Body time is the perception our minds and bodies have of the passing of time. The body's dominant time cycle is called the "circadian rhythm." This is controllled from our brain and is normally around 25 hours. Researchers looking at the difference between nature's 24 hour day and the body's 25 hour circadian rhythm have concluded the difference may have created a degree of tension which was necessary for survival in primitive times.
The study of relationships between time and the time cycles of the body is called "Chronobionics". Studies in this area indicate we respond to a number of weekly cycles in our body chemicals, immune-system, response and blood circulation. Such rhythms may explain the seven day week as a unit of time. This is the only calender measure that does not trace its origins to astronomy.
Research and human experience has shown that if we abuse our internal rhythms, circadian or otherwise, we are in for trouble. Monday-morning blues, sleep disorders and depression can result from the mismatch between clock on the wall, and the one in our body. Ask any international traveller about jet-lag and they will certainly back up the theory.
Further research in this area is looking at the exacerbation of the problems caused by this mismatch when drugs (eg. alcohol and nicotine) are introduced. Science can now confirm for us what we always knew about late nights.